Hum and noise reduction



Sept. 21, 1937. l DE Wn-T 2,093,751

HUM AND NQISE REDUCTION Filed Jan. 31, 1936 2 sheets-sheet 1v Jgd INVENTOR. J. H. DE WITT Sept 21, 1937. J. lH. DE WITT HUM AND NOISE REDUCTION Filed Jan. 51, 195e 2 Sheets-Sheet 2 INVENTOR. J H DE WITT ATTORNEY.

Patented sept. 21, 1937 n HUM NOISE REDUCTION UNITED STATES PATENroFFl-CE .ruim n. ne Witt, Nashviuarenm. Application January 31, 1936, Serial No. 61,634

ze claims. (c1. 25o- 17) My present invention relates to .the-` reduction of hum and noise in high frequency systems and particularly in radio transmitters.

Modern practice dictates, for economy, the use of commercial alternating'currents for energizing the cathodes and filaments of the tubes in the various stages of a radio transmitter. In addition, it has proven highly practical to polarize the plates of the vacuum tubes employed in transmitters with rectified low frequency lalter-- nating current. However, concomitant with the use of alternating current energlzationcircuits there appears in the output of the transmitters objectionable noise, usually in the form of hum correspondingl in frequency`to./the fundamental and one or-more harmonics of the low frequency energizing source. To reduce this hum it has been proposed, in addition to filtering, to

make use of polyphase filaments and to excite' them from a source of polyphase current. While such action has proven a partial solution of the problem; it nevertheless leaves much to1be desired, especially, for example, in connection with the utilization of present day high delity re. ceivers; for, with such receivers, -transmitter noise is still undesirably manifest.

An object of my presentinvention is,'therefore, to provide a noise and hum reduction system for high frequency circuits in generaland for transmitters in particular, which system-will reduce hum and noise to a much lower level thanth'at attainable heretofore, as far as I am aware. This object is effected by'employing feed-back circuits whichtend to cause a disproportionate reduction in hum and noise` generated throughout the various stages of the system,1which, in

the Acase of a radio transmitter, include theradio frequency stages.

The use of degenerative feed-back in active low frequency networks has for some time been recognized as a means of reducing] hum, noise and modulation products as Well as accomplish# ing other desirable results such as stabilizing 'the gain. 'Ihe application Orl this principle is relatively simple in audio frequency ampliiis in which there is a small amount of phase shift,l

but its application to a complicated circuit such as a radio transmitter` presents difiiculties, especially as regards oscillation production -vat an undesired, troublesome frequency.' Accordingly,

a further object of this invention is to'provide 'a simple, ecient and inexpensive h um, noise and distortion reducing circuit for radio transmitting appara-tus which circuit will not produce undesired osci-llations.

Briefiy; my present invention makes use of ka feed-back system which reduces undesired noise and distortion without generating an independent oscillation. For this purpose, an ideal feedback circuit would be one in whichthe phase does not shift appreciably over a frequency range from zero to infinity. Such a circuit wouldhave no tendency to oscillate with any amount of degenerative feed-back. In 'any circuit in which the phase around the feedback loop changes 18Ofoscillations will take place providing the 10 other wave path is combined with the first and 25 isarranged to have a gain when considered from the transmitter input to its output equal to or greater than the gain -in the transmitter or rst path. Moreover, the second wave path is so designed as to have verylittle phase shift over a A frequency range somewhat wider than that of th transmitter. The two paths are so connected that their output voltages would be in phase over the wanted frequency range. 'I'he voltage or current fed back to the transmitter is the vector sum ofthe voltages or, currents from the two paths, as a consequence of which at whateverl frequency the phase through the transmitter shifts 180j the feed-back is neutralized as the currents in the l two paths are, out of phase and thus singing is prevented. IIn a preferred form of my invention,

a simple filter is placed in the second. path so as v to allow its operation only near the frequenciesA at which oscillations would-take place. The rearr yson` forthis is to prevent degeneration of However, this degenerativeA ction throughthe second'path is very 'desirabe near the frequencies at which it is tuned, as it prevents unwanted distortion products which 50 would be nearly in phase lai: these frequencies -from beingamplifled by the transmitter.

'That isto say, my present invention employs ltwo wave paths of approximately equal gain. In a preferred modification, one'path is the trans- 55 e secgnd path over the wanted frequency rang of the transmitter.

' mitter has'a tendency to oscillate.

practice, simple filters have or other apparatus having'relatively little phase shift over the frequency range of the main path. 'I'he two paths are so arranged that their outputs are in phase over the wanted range and are out of phase at whatever frequency the trans- As a result, when the phase through the transmitter shifts 180 degrees from its normal condition, the feedback balances out and singing is prevented. In been used in the second path to allow it to operate only near the frequencies at which the transmitter tends to sing.

My present invention will be described in greater detail in conjunction with the accompanying drawings which are not to be considered as imposing any limitations upon it, for it is to be clearly understood that the drawings illustrate only some of the many possible arrangements employing the fundamental principles herein given.

In the drawings, Figures 1 and 2 are block diagrams illustrative of the general principles employed in my invention and Figure 3 lis a wiring diagram partially in block form of a complete radio transmitter employing circuits for reducing hum, noise and distortion accordingto my invention.

Referring to Figure 1, the signal to be broadcast coming in on the transmission line TL is fed to the radio transmitter RT and radiated over the antenna A. The radio transmitter RT includes audio, radio frequency generator, modulator, and power amplifier stages, which in and of themselves introduce.distortion due, among' other things, to imperfect tube and circuit characteristics. YIn addition, because of alternating currents employed to energize the cathodes inv the radio transmitter RT and due to the use of rectified alternating currents for supplying the anodes of the various tubes in the radio transmitter RT there will be present, in the voutput of the radio transmitter, undesirable hum which is radiated as one of the products of modulation over the antenna A; To reduce the distortion and hum components, a portion of the transmitter output is rectified in the rectifier R and fed back through hum amplifier HA in degenerative phase to the audio frequency input circuit of the transmitter RT. It will be found that that degenerative feed-back which causes an appreciable ref duction in hum is accompanied by a reduction in gain which may be made up for by an additional stage of audio frequency amplication, intheradio transmitter.l As thecost of such'astage of amplification is-relatively slight, compared to the cost of the entire transmitter'l and vthe advantages to be gained by the use of this invention, it is not objectionable.

However, the degenerative feed-back whichl causes appreciable reduction in distortion and' hum is also accompanied by parasitic oscillation generation at some undesired frequency above or below the usable modulation band of the transmitter. This occurs because the rectifier R and the Justed in combination with the radio transmitter RT to produce degenerative effects over the usable. band ofY frequencies, operate regeneratively at parasitic frequencies for4 which the integrated phase shifts of the stages .in the radio. transmitter, the -rectifier R and the amplifier HA produce the Acondition or overall 180 -phase shift essential for regenerative action.

` is simplied to distortion and hum amplifier HA, while adbythe commercial electricity'supply source. Y

-mitter Aitself and the other'a simple amplier The foregoing defect is remedied by providing an additional network NW which produces zero phase shift over a range including the modulation range of frequencies and which produces essentially a 180 degree phase shift in parasitic frequency oscillations so that at the combining point CP the parasitic oscillations fed back into the transmitter are reduced to such an extent as not to produce undesired singing or parasitic oscillation generation.

Figure 2 is afsimplified arrangement of the apparatus diagrammatically shown in Figure 1 and the extent that the need for a separate degenerative amplifier HA is eliminated. This is accomplished by connecting the network NW, as shown, to one of the lower audio frequency stages of the radio tansmitter.

Figure 3 is a wiring diagram of a preferred manner in which theprinciples of my present invention may be carried into effect and illusamplification .ASL This stage includes the two.

vacuum tubes l, 6 whose outputs are fedthrough transformer 8 to the modulator I0. The modulator IU may ,be of any of the conventional types and serves to modulate the high frequency oscillations generated at the high frequency oscillator HFO and fed 'to the modulator I0 through the buffer amplifier BA.{ The buffer amplifier, if desired, may contain frequency multipliers so as to produce the desired radiation frequency from a lower frequency high frequency oscillator. The output of the modulator I0 is fed through lthe radio frequency power amplifier I2 whose output in turn is radiated from the antenna A.

'I'he filaments 'or cathodes for the tube stages ASI and A52 as well as for the tube AT areenergized from a suitable source of supply such as a 60 cycle commercial transformer Il, provided formers I8, 20,'the midpoints of the secondaries of which are grounded, as illustrated. In addition, by-passing condensers22, 24 are provided. .The'by-passing condensers 22, 24 and the midtap connections to the secondaries of the transformers, redu to some extent hum due to application of alternatipg currents to the lfilaments or cathodes of the tubes employed.

The buffer amplifier BA, the modulator I0 and radio frequency power amplifier I2 'are supplied with, for example, 60 cycle commercial electricity from a three phase line 26. As shown, a different phase is connected to each of the oscillator and buffer amplifier, modulator and power amplifier stages. This will not only serve to equalize the load on the power supply, but will also tend to' cause a reduction in hum. If desired, three phase or, in general, polyphase filaments may be employed in lator and power amplifier stages so as to further lessen lthe ripple introduced into the final output the buffer amplifier moduof such value asto produce objectionable noise in, for example, high delity receivers. The reduction of this hum to a negligible value according to my presentiinvention is accomplished in the following manner.

A rectifier, here shown as of the full wave type employing vacuum tube rectiers or diodes 28,

30, Ais provided with a tunable input circuit 32 coupled to theoutput of the power amplifier I2. The rectified output of the rectifier, which is a replica of the modulation applied to point TL as modified by the distortion and hum and noise through the transmitter including stages ASI, AS2, I0, I2, etc. is fed through resistor 3ft, am-

meter 36 and, by-passing'condenser 38 across the conjugate points 40, c2 of the resistance bridge RB. As a consequence the rectified current is unable to produce any reaction, or in other words,

is not fed back over the transmission line TL to` cause distortion, but is simply applied to the control grid 40 of the vacuum tube stage ASI.V

Similarly, it is to be noted that by virtue of the connection of the secondary of transformer TF to the conjugate points 46, t8 no voltage will be impressed upon the rectifier R., but only upon the grid electrode lill of tube ASI.

Attention is directed to the fact that the tubes of each stage of the transmitter and the coupling circuits thereof do not introduceany appreciable phase shift in the alternating currents .passing therethrough tothe antenna where the alternating currents under consideration are, in a properly designed transmitter, within a range ofl frequencies including the usable modulation range.

This condition is not affected by the rectifier R, whc-e voltage output appearing across resistor 50 is fed back through the large condenser 384 to the grid 4d of stage ASI. The .feed-back through the condenser 38 is in degenerative phase so that the signal, distortion and hum at the' output of the power amplifier i2 are reduced. This loss in signal is made up for by the inclusion of a stage such as ASI which otherwise might not be present in the transmitter. It will be found that with this arrangement the hum and distortion level in the output of the transmitter is reduced, but as this hum level is further reduced by increasing the degenerative feed-backlit will be found that undesirableparasitic oscillations are set up.

These parasitic oscillations are set up because of the fact that at points beyond the'frequency range for which the transmitter does not produce phase shifts, each stage does introduce, at the parasitic frequencies, a certain phase shift depending upon the tubes and circuits employed. The integrated phase shift throughout the stages is such that for -frequencies lying outside of the usable frequency range the otherwise degenerative feed-back becomes regenerative, causing the undesired oscillation generation.

This defect is remedied .according to my present inventionb'y providing another path from a lower stage of the transmitter, such as stage ASI, which will. cause such cancellationA of parasitic regeneration as to eliminate singing or oscillation at parasitic frequencies. This additional path, as illustrated in Figure 3, includes the conductor 60,

by-passing condenser 62, resistance 64 to prevent short circuiting of the audio output of the stage A ASI and circuits 66. 68 connected as shown. Usually it will be found that' parasitic oscillation generation takes place at frequencies above the modulation range' of the transmitter. Accordingly, the circuit 66, connected as shown, is provided which is made resonant above the working lrange ofthe transmitter. The effect of this circuit is to introduce a voltage substantially equal and opposite to the parasitic voltage set up across the resistor 50. As a consequence, a parasitic voltage of no appreciable consequence is fed back through condenser 38 and is of insuicient'value to cause parasitic oscillation generation at the upper range of frequencies. This phase opposition effect is produced by virtue of the tuning of the circuit 66 and the value of the neutralizing voltage may be adjusted by, for example, adjustment of resistor 64. l

In a similar way circuit 681s connected to the path including condenser 62- and resistor 64, but

` circuit 68 is tuned below the lower range of modulation frequencies and is added Vwhere there is a tendency for generation of parasitics at the lower frequencies producing an eifect commonly known as ,motor boating.

One of the actual circuits set up embodying the principles of my present invention was as follows:

In a standard 5 kw. transmitter an audio amplier stage utilizing two #53 tubeaconnected as a phase inverter was added 'aheadof the audio system of the transmitter. Apush-pull rectifier employing two v#1V tubes was coupled to the transmitter output so as to provide audio feed# back voltage. A second feed-back path was provided in the 1form of a tap from the plate resistor 'I0 of the first audio stage ASI. The circuit of the audio stage, the feed-back rectifier, andthe feed-back neutralizi'ngpath which were'added to the transmitter were as shown in Figure 3. It will be observed that the second path is in theform of a tap from the vplate of the first audio stage and current flows through this path andis fed into a parallel resonant circuit, the lower side of which is grounded: In this transmitter it was only necessary to neutralize a frequency of about 12,000 cycles because of the fact that the phase shift at the low end vof the frequency range is rather small at appreciable amplitudes. The circuit diagram shows that the outputof the feed-back rectifier which appears across 50 is connected inseries with the voltage that would be built up across the tuned circuit near the singing frequency. v i

It is to beemphasized that this actual setup as generally illustrated by Figure 3, is only onel of the many possible arrangements of the fundamental principle of providing two wave paths.

Measurement of the transmitter by means of a cathode ray oscilloscope indicated that the phase shift'throligh the entire system; that is, from the grid of the rst tube ASI to the output of the feed-back rectifier R when the second path was short circuited was at about 7,000 cycles, and at about 12,000 cycles. Observations on feed-back systems indicate that distortion and noise components whichare fed back at a phase relation greater than 270 are likely to add to the totaldistortion and in this transmitter-it would .be expectedthat any distortion component lying above 7,000 cycles will be increased by' the use of feed-back. Also, the gain reduction above this frequency 'will be less than at lower frequencies where the feed-back is 180 out of phase. This would cause the overall frequency characteristic Y to riseabout 7,000 cycles. One of the features may be so adjusted as to provide an excess of neutralizing voltage inl the region of the "singing frequency, and so lower first stage by degenerative feed-back action that the frequency characteristic may be made very uniform. This adjustment at the sametimeacts to degenerate high order distortion components which ywould ile in this` case above '1,000 cycles and so prevents their addition to the-transmitter distortionv in thisf range. The frequency characteristic may be used-as a criterion for this adjustment and, of course, it is desirable that it be uniform.

Another means of ysecuring the same result in a laboratory set-up' is to measure the frequency characteristic with the feed-back loop open at the input to the first tube.' In this case an audio oscillator of constant output would be fed in at this point. A cathode ray oscilloscope may be connected so as to indicate the phase and amplitude relations existing between the oscillator voltage and the voltage appearing vacross the combination of the two paths. In this case the secqnd path is adjusted both in' phase andin amplitude so that the amplitude and phase characteristic is uniform or reasonably s o over the wanted frequency band. When this adjustment is completed, the feed-back loop may -be joined and the frequency characteristic of the transmitter will be found uniform. l

Having thus described my invention, what I claim is:

1. -In a radio transmitting )system wherein audio frequency waves are amplifled,`wherein radio frequency waves are generated, wherein the amplified audio frequency waves are used to amplitude modulate quency waves and wherein an undesirable hum component is present in the modulated waves due Ato energization of tubes in said transmitter with currents derived from a low frequency alternating currentsource, the method of reducing the hum component'which includes rectifying the portion of the modulated waves so as to obtain a copyfof the audio frequency waves and hum introduced at the transmitter, lfeeding back the rectified waves for reamplification Wlththe audio frequency waves sc that the hum compo--Y nent is appreciably reduced and degeneratively combining parasiticr frequency waves-with waves. fed back for hum reduction so as tol prevent un'- desirable parasitic @oscillation generation at freq`uencies beyond the usable Afrequency range of the transmitter. r

2. In a radio transmittingsystem for trans-i mitting amplitude modulatedwaves with a'substantial reductionx in hum and distortion, an audio frequency amplifier; a radio frequency generator, a modulator, meansv for feeding waves from said generator and amplifier to said modu s lator, apower amplifier coupled to saidmodulator and serving to( amplify amplitude modulated L waves derived from said modulator, means for energizing the cathode ofa tube in said power amplifier with currents derived from a low Vfre-- quency alternating current source, means for supplying the anodedof a tube' in said power amplifier with a rectified current derived from a low frequencyv alternating current source,the excitation of said tube serving to introduce undesirable hum and distortion in the 'output of said power amplifier, means for reducing said hum and distortion components comprising a rectifier the gain of the` the generated radio fre-l range of the transmitter.

.Jugate ,ossnsii of this system 1s that the-second path u, u, .is

coupled to the output of said power ampliner and means for feeding back in reversed phase to said audio frequency amplifier rectified currents derivedf from said rectifier whereby hum and distortion are reduced with concomitant tendency to produce, parasitic oscillations at frequencies beyond the usual audio frequency range', and means for preventing the generation of parasitic oscillations comprising a circuit connected between said' audio amplifier and the rectifier for feeding into the rectiner feed-back path a voltage which-is cophasai with the rectified voltage over a range including the useful audio frequency range and for -feeding into the rectiner feed-back circuit avoltage which is substantiallyl equal and opposite in phase to parasitic frequency voltages appearing in the output circuitcf said rectifier which would otherwise tend to produce undesired parasitic oscillation generation.

3. Apparatus acclaimed in claim 2, characterined by the fact that each of the stages of the transmitter contains a tube whose cathode is energized with currents derived from a low frequency alternating current source and whose anode is supplied with voltage derived yfrom a rectified low frequency alternating current. and being further characterized by the fact that the rectifier output is fed back tol an early point in theaudio frequency' system of the transmitter and that a feed-back path for'preventin Darasitic oscillation generation is connected to a, later point 'in the audiofrequency system and being further characterized by thev fact that the parasitic oscillation generation prevention path includes a resonant circuit resonant at some frequency beyond the working audio rangeof the transmitter.

4.-In a radio transmittingsystem wherein audio frequency waves are amplified, wherein radio frequency waves are generated, wherein the' amplified audio frequency waves are used to amplitude modulate the generated radio frequency waves and wherein distortion components are present in the modulated waves, the method of reducing the distortion components which includes rectifying a portion ofthe modulated waves so as to obtain a u. copyof the audio frequency waves and distortion. feeding' back the rectified waves for reampliiication with the audio frequency waves so that the distortion components' are reduced and degeneratiiyely combining parasitic frequency wavesfwith waves fed jfordistortion reduction so as to preventvundesirable parasitic oscillation generation at frequencies beyond the usable frequency 5. In combination. a source of modulatins waves,'a source of carrier waves, means for modulating waves from said carrier wave source with waves from said modulation wave source, means for feeding said waves to an output circuit, a 'rectiiier for'rectifying a portion of lthe modulated waves appearing lin said output circuit, means for' feeding the output of said rectifier in degenerative phase into "said source cfmodulating waves so that' distortion products appearing in the output ycircuit are reduced, and means for feeding waves of y distortion frequenciesv into said feed-back means in suchphasc as to substantially eliminate regeneration effects at distortionfrequencies.

6 -In (combination, a bridge circuit, means for feeding modulation potentials .across two conpoints of said bridge circuit, a modulation ampliner connected bridge, means utilizing the output of said modulation amplifier to modulate carrier waves. means 'unsymmetrically to 'said waves, means for'feeding the rectified carrier waves in' degenerative phase to the other pair of conjugate points of said bridge, and means, asso ciated with said last mentioned means', for preventing excessive unwanted regeneration at one or'more frequencies at which excessive unwanted regeneration occurs. 'V v 7. Apparatus as claimed in cia-im 6,'characterl-ized by the fact that the means for preventing excessive regeneration includes a tuned circuit tuned to a frequency at which excessive regeneration would otherwise occur.

8. In a radio transmitting system having a modulation frequency amplifying circuit and av modulated high frequency output circuit, a rectier coupled to said output circuit for rectifying a 'portion of the wave energy appearing therein, a

feed-back circuit for feeding back the rectified energy to said modulation circuit, thefeed-back being so adjusted `as to be degenerative in phase and so as to reduce distortion components in said output circuit, ,and means connected with said feed-back means forvminimizing and substantially preventing regenerative action at distortion frequencies whereby the energy in 'said output -circuit is substantially confined in frequency to a desired range of frequencies.

9. In a radio transmitter having an audio frequency ampliiierftube and an output circuit in which modulated highl frequency Waves appear, an audio frequency bridge circuit, means for coupling input electrodes of said amplifier unsymmetrically to said bridge circuit, means for applying audio frequency/waves to a paix' of conjugate points on said bridge circuit, a rectifier coupled to said output circuit, a resistance connected between the output electrodes of said rectifier, means including a condenser for applying the voltage drop across said resistance acrossl the other pair of conjugate points of said bridge circuit, and a tuned circuit connected between terminals of said resistance for introducing into said resistance voltages opposing voltages in said resistance, which latter voltages tend to cause undesired eiiects in said radio transmitter.

10.y In a radiotransmitter having an audio frequency amplifyingtube and an output circuit in which modulated radio frequency energy appears and circuits operatively connected between said audio frequency tube and said output circuit, a bridge circuit, means coupling the input electrodes of said audio frequency amplifier to one arm of said bridge circuit, means coupling audio frequency waves to be transmitted to a pair of opposite corners of said bridge, a rectier coupled to said output circuit, a resistance connected between the output electrodes of said rectifier, means for feeding' the voltage dropl across said resistance in degenerative phase to the vother opposite corners of said bridge, said means including a serially connected condenser, thel arrangement as so far described being adjusted so vthe output circuit of said rectifier, said connecting circuit including a condenser, a resistance, and a circuit, connected across said degenerative feeding means, tuned to a Afrequency beyond the desired operating range.

oscillation generationI is connected to a later point in the audio frequency system and being further characterized by the fact that the parasitic oscillation generation prevention path includes a circuit of high impedance at some frequency beyondv theworking audio range of the transmitter.

12. In combination, a source of modulating waves, a source of carrier waves, means for modulating waves from said carrier wave source with vwaves from said modulation wave source,- means for feeding said waves -to an output circuit, a rectifier for rectifying a portion of the modulated waves appearing in said output circuit, means for feeding the output of said rectifier in degenerative phase into said source of modulating waves so that distortion products appearing in the output circuit are appreciably reduced, and means, connected to the foregoing apparatus, selectivelyresponsive to and reducing feed-back at one or more frequencies at which the integrated phase shift through said feed-back means vis undesirably regenerative.

13. Apparatus as claimed in claim 6, characterized by the fact thatthe means for preventing unwanted regeneration includes a circuit having high impedance to a frequency at which the unwanted regeneration occurs.

14. In a radio transmitting system having a modulation frequency-amplifying circuit and a modulated high frequency output circuit, a rectier coupled to said output circuit for rectifying a portion of the wave energy appearing therein, a feed-back circuit for feeding back the rectified energy to said modulation circuit, the feed-back being so adjusted as to be degenerative in phase and so as to reduce distortion components in said output circuit, and means connectedwith the` foregoing apparatus for preventing excessive un 15. In a radio transmitter having an audiofrequency amplifying vtube and an output circuit in which modulated radio frequency venergy appears and circuits operatively connected between said audio frequency tube and said output Y circuit, a bridge circuit, means coupling the input electrodes of ,'said audio frequency amplifier to one arm of said bridge circuit, means coupling audio frequency vaves to be transmitted to a pair of opposite corners of said bridge, a rectifier coupled to said output circuit, a resistance connected btween the output electrodes of said rectifier; means for feeding the voltage drop across said resistance in `degenerative phase to thef 4other opposite corners of said bridge, said means including a serially connected condenser, the arrangement as so far described being adjusted so as to cause degeneration of distortion components appearing in said output circuit but having unwanted regenerative amplification at certain frequencies; and means for reducing said unwanted regenerative amplification at saidfrequencies put circuit,

comprising a circuit including a condenserand a resistance connected from an output circuit of said audio frequency ampliflerto the output cir- `cuit of said rectifier, and a saiddegenerative feeding means, said last-mentioned circuit having high impedance to a' frequency at which unwanted regenerative amplicircuit connected to fication occurs.

16. In combination, a source waves, asource of carrier waves, means for modulating waves from -said carrier'wave source with waves from said modulation wave source, means for feeding the modulated waves to an outa'rectifler for rectifying a portion of the modulated waves to reproduce waves corresponding to the modulation, means for providing'feed-b'ack of the produced modulation waves to the source of 'modulating waves in order to reduce distortion produced in the normal modulation frequency band, and means, connected with selectively responsive to the foregoing apparatus, a frequency at which feed-back may be so shifted in phase as to produce excessive regenerative amplification, .for preventing said' excessive regenerative ampliiication.

1'7'. Apparatus as claimedl in claim 16, charac-` terized by the fact that said selectively responsive means comprises aresonant circuit.

1B. In combination, a source of modulating waves, a source of carrier waves, means for modulatingwaves from said carrier wave source with waves from said modulation wave source, means for lfeeding I the modulated waves to an output circuit, a rectier for rectifying a portion `of the modulated waves to reproducefwaves corresponding to the modulation, means f or providing feedback ofthe produced modulation waves to the source ofmodulating wavesl in Vorder to reduce distortion produced in the normal modulation frequency band, and means,- connected with the foregoing apparatus, comprising a circuit of high impedancefor frequencies above the modulation band of frequencies for reducing the feedback for frequencies outside the normal modulation frequency band where the feed-back may be so shifted in phase as to become regenerative instead of degenerative in its effect.

19;--Incombination, a' source of modulating waves, a source of carrier waves, means for modulating waves from said carrier wave source with waves from said modulation wave source, means for feeding the modulated waves to an output circuit, a rectifier for rectifying a portion of the modulated waves to reproduce waves corresponding to the modulation, means for providing feed-back of the produced modulation waves to the source of modulating waves `in order to reduce distortion produced in the normal modulation frequency band, and means, connected to the foregoing apparatus, comprising a circuit,

where the feed-back may be so shifted in phase as to become' regenerative instead of degenerative in its effect, to prevent excessive regeneration.

20. Apparatus as claimed in claim 19, characterizedv by the fact that said selectively responsive circuit comprises a resonant circuit.

21. In combination,- waves, a' source of carrier waves, means for modulating waves with waves. from means for feeding the modulated waves to an output circuit', a rectifier for rectifying a portion of the modulatedwaves to reproduce waves correspondingto the modulation, means for providlof modulating 5 its 656%.

-a source of modulating from said caTier wave source said modulation wave source,

to the source of modulating waves lation frequency band, and means, connected to the foregoing apparatus, comprising a circuit j of high impedance below the modulation band of frequencies feed-back for frequenciesoutside the normal modulation frequency band where the feed-back may be so shifted in phase as to become regenerafor frequencies' tive instead of degenerative'in its' effect.

a source of ymodulating 22. In combination, waves, a source of carrier waves, means for modulating waves -fromsaid carrier wave source with waves from said modulation wave. source, means for feeding the modulated waves to an output circuit, a rectifier for rectfying a portion of the modulated waves to reproduce waves corresponding to the modulation, means for provid- -ing feed-back of the producedmodulation waves to the source of modulatin'g'waves in order to reduce distortion produced in the normal modulation frequency band, Vand means, connected to the foregoing apparatus, comprising a circuit of high impedance for frequencies above and below ,the modulation band of frequencies for reducing the feed-back for frequencies outside for reducing the the normal modulation frequency. band where-r the feed-back may become regenerative be so shlfted'in phase as to insteadof degenerative in 2 3. A loop system ulating waves, a sourceof carrier waves, means for modulating waves from said carrier wave source with waves from Ysaid modulation wave source, means for feeding the modulated waves to an output circuit, are'ctierfor vrectifying a portion of the modulatedf'waves to' reproduce waves corresponing to. thejfifiodulationfmeansl for providing feed-back of the ,producedmodulation waves to the source of modulating waves comprising-,Ia source ofmodin order to reduce distortion produced in the normal modulation frequency band, and means connected with said loop systemc comprising va circuit of suitable impedance for reducing the feed-back for frequenciesv `where the feed-back.

may be so shifted in phase 'through said loop system as to become regenerative instead o f degenerative in its effect.

"24. In wave transmitting' apparatus, a loop Vsystem comprising a modulation wave`.amplifier,

a source of high frequency oscillations, a modulator for combining wavesfrom said amplifier y and oscillator'toproduce modulated high frequency waves, a demodulator for demodulating aportion of the modulated wavesand means degeneratively feeding back demodulated waves to the modulation wave amplifier to reduce distortion; and a feed-back-circuit connectedto a portion of the'modulation wave amplier for reducing the gain of the loopsystem a t at least one frequency for which -the-phaseshift around the loop system is undesirably regenerative.

25. In wave transmitting apparatus, 'a loop system comprising a modulation wave amplifier, a source of high frequency oscillations, a modulator for combining waves from said amplifier and oscillator to produce modulated. high frequencyv waves, a demodulator for .'demodulating a -portion of the modulated wavesV andmeans for -degeneratively feedingy back 'demodulated waves to the modulation waveampliiier to reduce distortion; and a resonant circuit lconnected to asfto reducevfeedback for at -le'astone frequency for which the integrated the loop system so spaans-r e '7 phase shift around the loop system produces unwanted regeneration.

26. In wave transmitting apparatus, a. loop,

system comprising Aa, modulation wave amplier, a source of high frequency oscillations, a modulator for combining waves from said ampliierand oscillator to produce modulated high frequency waves, a demodulator .for demodulating a portion of the modulated waves and means for degeneratively4 'feeding back .demodulatedv waves to the modulation wave amplifier to reduce distortion; and a. parallel tuned circultconnected to the loop system so as'to reduce feedback for at least oneyfrequency for which the integrated phase shift aroundV the loop system produces unwanted regeneration. l JOHN H. DE WITT. 

